Equalizer design has long been one of the most important considerations in the design of receivers suited for providing modern digital land-line-based data services such as, for example, DDS and T1. Both of these services use bipolar return-to-zero ("BRZ") signals for transmission. As is known, in a BRZ transmission system a "1" logical value is transmitted as either a positive or negative pulse while a "0" logical value is denoted by the absence of a pulse. Successive pulses alternate in polarity, giving rise to the term "alternate mark inversion," or "AMI." Certain conditions cause this rule to be violated but, under this rule, it is never legal to transmit two consecutive positive or negative pulses.
Conventional equalizers for BRZ signals operate by selecting an appropriate inverse line model for the given communication channel. If the line model is correct, the attenuation and phase distortion introduced by the line can be effectively compensated for in the received signal. A noise limiting filter is sometimes added as well to eliminate out-of-band noise.
The problem with these conventional equalizer structures is that their performance is limited by the accuracy of the line models. Impairments such as bridge taps and wire size transitions sometimes cause a line to have characteristics that are not predicted well by normal wire line models. One solution to this problem would be to generate line models that take into account every known line impairment combination. It is easy to see that this approach becomes impractical quickly as more and more impairment sources are considered. A better approach is to build a receiver structure that is capable of learning the line impairments and compensating for them.